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Due to their ability to provide more uniform story drifts with building height, strongback braced frames (SBFs) have been proposed as enhanced-performance structural systems. Conventional design approaches, however, tend to underestimate force demands in strongback elements compared to nonlinear response history analysis (NRHA). To address this gap, alternative methods such as modal pushover analysis (MPA) have been suggested to obtain less computationally intensive estimates of seismic demands. This study presents a statistical assessment of MPA as an estimate of NRHA force demands for an 8-story SBF subjected to 44 far-field ground motion records scaled at the risk-adjusted maximum considered earthquake (MCE𝑟 ) intensity level. Unlike prior studies that compare MPA results to the statistics of the NRHA response, this work treats each ground motion as a separate test to characterize how MPA accounts for record-specific spectral characteristics. Accuracy in estimates of the force demands (i.e., how close the MPA estimates are to the NRHA “truth”) is characterized using root mean square error. Additional comparisons are made across the MPA parameters, such as the number of modes employed, as well as the use of initial versus elongated periods. Results provide a comprehensive statistical assessment of MPA, illustrating that the approach can be sensitive to spectral assumptions and is better suited to aggregated estimates from NRHA. 

Concentration of drifts due to story mechanisms can lead to severe structural damage and economic loss. Frame-Spine systems have been proposed to mitigate these effects by redistributing drift demands with building height; however, systems can also exhibit near-elastic higher-mode effects, resulting in forces and floor accelerations that remain largely unreduced by inelastic behavior, thereby adversely affecting acceleration-sensitive nonstructural components and occupants. To address near-elastic higher-mode effects, Force-Limiting Connections (FLCs) have been introduced limiting force transfer between the frame and the spine and reducing acceleration demands through controlled yielding components. This study presents observations from full-scale shake-table testing of a four-story Frame-Spine and a Frame-Spine-FLC specimen at E-Defense. Results highlight higher-mode effects under strong shaking, with emphasis on (1) story shear resisted by the spine, (2) force–deformation behavior of the spine-to-frame connections, and (3) vertical distribution of forces. These findings provide experimental evidence of higher-mode participation in Frame-Spine systems and support the development of improved design guidance and controlling mechanisms. 

A novel structural system is being developed collaboratively by researchers from the United States and Japan to protect essential facilities, such as hospitals, where damage to the building and its contents and occupant injuries must be prevented and where continuity of operation must be maintained. The development is focusing on new construction, but it also has potential for use in seismic retrofit of deficient existing buildings. The new system employs practical structural components, including (1) flexible steel moment frames, (2) stiff steel elastic spines and (3) force-limiting connections (FLC) that connect the frames to the spines, to economically control building response and prevent damaging levels of displacement and acceleration. The moment frames serve as the economical primary element of the system to resist a significant proportion of the lateral load, dissipate energy through controlled nonlinear response and provide persistent positive lateral stiffness. The spines distribute response evenly over the height of the building and prevent story mechanisms, and the FLC reduce higher-mode effects and provide supplemental energy dissipation. The full-scale shake-table testing of a building with the Frame-Spine-FLC System, which represents a hospital facility and includes realistic nonstructural components and medical equipment, validated the functionality of the structural system.